This project investigates the expression of proto-oncogenes during the differentiation of embryonic lens epithelial cells to form lens fiber cells, and seeks to determine the specific function of the corresponding gene products in the developing lens. Measurements of steady state mRNA levels and nuclear run-on transcription experiments have identified several proto-oncogenes which are actively expressed in the embryonic lens. Among these are the nuclear proto-oncogenes, c-myc, c-fos, and p53, and the membrane- associated tyrosine-specific protein kinase, c-src. A transient increase in the expression of the c-myc gene which occurs as the differentiating cells withdraw from the cell cycle suggests that this proto-oncogene may regulate some aspect of differentiation. The increased expression of c-myc has been shown to be primarily post-transcriptional, although a small increase in transcription has also been observed. The increase in c-myc expression which occurs during differentiation can be mimicked pharmacologically by agents which block the lipoxygenase pathway of arachidonic acid metabolism. Increased levels of c-myc mRNA, whether in differentiating cells or in cells treated with lipoxygenase inhibitors, are correlated with accumulation of mRNA for the heat shock protein, HSP70.